Automatic xerographic development control



April.l5,l969 I (KING 3,438,705

AUTOMATIC XEROGRAPH'IC DEVELOPMENT CONTROL .Fil edqan. s, 1967 Sheet ofs POWER SUPPLY TO ELECTRODE I-J 76 INVENTOR.

' I PAU 5mm; FIG4 I v A T TORNEYS April 15, 1969 P., F. KING AUTOMATICXEROGRAPHIC DEVELOPMENT CONTROL Filed Jaxi. 5,1967- Shee t 3 of3 MGRINVENTOR. PAUL F. KING M A T TOR/VEVS' April 15, 1969 P. F. KING3,438,705

. AUTOMATIC XEROGRAPHIC DEVELOPMENT CONTROL Filed Jan. s, 1967 Sheet 3of 5 54 r 7 'roamcruws i ELECTRODE coN'moL.

' l PHOTOMJLTIiLER FIG. 7

IIVNT.

AL? ms ATTORNEYS United States Patent 3,438,705 AUTOMATIC XEROGRAPHICDEVELOPMENT CONTROL Paul F. King, Webster, N.Y., assignor to XeroxCorporation, Rochester, N.Y., a corporation of New York Filed Jan. 3,1967, Ser. No. 606,888 Int. Cl. G03g /04 US. Cl. 3558 9 Claims ABSTRACTOF THE DISCLOSURE A xerographic exposure and development apparatuswhereby background density is automatically controlled by aphotosensitive device which scans the material to be reproduced. Thepotential derived from scanning the background material by thephotosensitive device is then applied to the developing plate duringexposure thereby reducing overcharging of the plate.

Xerography, as pertinent to the present invention, comprises an imagereproduction method wherein an electrostatically charged photoconductiveinsulating member is exposed to a light image and the resultingelectrostatic latent image is developed or made visible through theselective deposition of electrostatically attractable particles.Alternatively, the latent image may optionally be transferred and fixedin image configuration to a sheet of paper or other support material.The development methods in widespread use in xerography were selectedfor their suitability for producing high contrast black and white copieswith a fixed exposure despite variability in the original document.However, development methods are also known which produce high qualityimages with solid area coverage tonal gradation. These methods resembleconventional silver halide photography in that exposure is preferablyadjusted to correspond to a particular subject or class of subjectsbeing reproduced. These methods are generally characterized, in that thephotoconductive insulating member, referred to as a xerographic plate,is brought into contact with electrostatically attractable particleswhile spaced as closely as feasible to an equipotential member known asa development electrode. In one known embodiment, the developmentelectrode is a metallic plate and the electrostatically attractableparticles are poured through the channel between the electrode and the'photoconductive insulator. In other known embodiments, the attractableparticles are precoated on a metallic plate which is then pressedagainst the photoconductive insulator or else the equipotential memberis comprised of electrically conductive filaments or the like mixed withelectrostatically attractable particles and poured across thephoto-conductive insulating member. These configurations cause anelectrostatic field to be formed between the photoconductive member andthe equipotential member in proportion to the charge on thephotoconductive member and are also eifective to increase the electricfield above large areas of uniform charge density. It is these electricfields which cause the electrostatically attractable particles to moveto and adhere to the photoconductive insulator for purposes ofdevelopment. In this way, large solid areas as well as tonal gradationsmay be developed. However, the potential on the development electrodemust be accurately matched to the minimum potential on thephotoconductor if images are to be formed with clear backgrounds orhighlights.

The exposure latitude is quite small when a development electrode isemployed as described above. The small exposure latitude on theunderexposed side of optimum is caused by the electrostatic potentialwhich remains ice on the plate in the background or highlight areas,i.e., those receiving maximum illumination. This background potentialproduces an electric field between the photoconductive insulator and thedevelopment electrode and the electrostatically attractable particlesare deposited in those areas giving a high background density in areaswhich should be reproduced as white. The small exposure latitude on theoverexposed side of optimum is caused by the reduced potential on thephotoconductive insulator in the image areas. This reduced potentialcreates an electric field which is insufficient to give good solid areadevelopment.

Brief summary of the invention Increasing the exposure latitude on theoverexposed side of optimum exposure can be achieved by minimizing thespace between the development electrode and the photocond'uctiveinsulator. For example, the previously described method may be used inwhich the electrode is coated with a thin layer of particles and pressedagainst the photoconductor. This invention, however, is primarilyaddressed to the problem of increasing the exposure latitude on theunderexposed side of optimum and, at the same time, provide an automaticcontrol to achieve minimum background density. While it is, inprinciple, possible to measure the potential on a xerographic plateafter exposure and adjust the electrode potential to the minimummeasured potential, such a method is not feasible in practice for tworeasons. First, the plate would have to be completely scanned by a verysmall area electrometer. This would be both expensive and timeconsuming, the plate potential meanwhile decaying as the measurement isin progress. Second, a particular subject may not contain a white area,and the minimum plate potential may not correspond to an area whichshould remain undeveloped. The desired result is accomplished bymeasuring the exposure which would be received by the xerogra phic platein a background or highlight area, deriving from this exposure and fromthe initial potential on the xerographic plate a new potentialsubstantially equal to the potential which would result from theexposure of the xerographic plate to a background or highlight area, andapplying this potential between the development electrode andxerographic plate during development.

Objects It is accordingly the principal object of the invention toprovide means and method for automatically optimizing the potential on axerographic development electrode so as to minimize backgrounddeposition in a developed image. Further objects will become apparent inconnection with the following description of various embodiments of theinvention.

Brief description 0 the drawings FIG. 1 shows a first embodiment ofexposure and development apparatus according to the invention;

FIG. 2 shows a modification of FIG. 1;

FIG. 3 shows a further embodiment in which the initial potential ismeasured before each exposure;

FIG. 4 shows a further form of control and development apparatus;

FIG. 5 shows a modification of FIG. 4 in which the initial potential ismeasured before each exposure;

FIG. 6 shows a further embodiment including an electronically adjustableresistor; and,

FIG. 7 shows the resistor of FIG. 6.

Detailed description FIG. 1 shows an illustrative embodiment of theinvention. A support member 10 is adapted to support a xerographic plate12 illustratively comprising a metallic backing sheet 14 and a layer ofphotoconductive insulating material 16, such as vitroeous selenium or adispersion of zinc oxide in a resin binder. A platen 18 is positionedfacing the xerographic plate and is adapted to carry a document,photograph or other subject 19 which is to be reproduced. When theplaten is illuminated by lamps 20, lens 22 will focus an image of thesubject onto the xerographic plate. There is also provided a developmentelectrode 24 which pivots about hinges and is illustrated in its upwardposition which permits exposure of the xerographic plate. Thedevelopment electrode can also be swung downwards (as positioned inFIGURE 3) where it defines a narrow channel with the xerographic plate.In this position, a xerographic developer, such as a mixture ofmicron-sized resin particles with larger glass beads or the like, can bepoured through the gap between the xerographic plate and the developmentelectrodes and caught in a receptacle 26. Alternatively, a gaseoussuspension of sub-micron sized particles can be blown through the spacebetween the xerographic plate and the development electrode or thedevelopment electrode can be precoated with particles and pressedagainst the plate.

The first step in making an image normally consists in electrostaticallycharging xerographic plate 12 to a uniform potential on the order ofseveral hundred volts. This can be carried out by any suitableconventional charging apparatus, not shown. When the xerographic plateis suitably charged and is in the indicated position, lamps 20 may beenergized for a suitable length of time required .to selectivelydischarge the charge on the xerographic plate to form a developableelectrostatic latent image pattern. Alternatively, the exposure can beadjusted by varying the light intensity or by varying the lens aperturethrough the use of a diaphragm 25. Since the density and quality of theresulting developed image is strongly dependent upon exposure when adevelopment electrode is employed, the exposure will be adjusted for theparticular subject matter or class of subject matter being reproducedthrough the use of an exposure meter, or trial and error, or thejudgment of the operator. Before starting the exposure, switch 28 willbe placed in the lefthand position and a photomultiplier or otherphotosensitive device 30 will look through lens 22 at a small sample ofbackground reference material 32, positioned on platen 18 adjacent tothe subject 19. Material 32 will be of the same material or at leasthave the same optical density as the background areas of the subject.Thus, the light incident on photomultiplier 30 will be the same as thatwhich would be received by an area of the xerographic plate upon whichis imaged a white or highlight area of the original subject.Photomultiplier 30 is connected to a voltage amplifier 34 which is, inturn, connected through a resistor 36 to capacitor 38. Capacitor 38 isthus charged up during an exposure in the same proportion as thexerographic plate is discharged. More specifically, the use of a highgain amplifier and a high value of resistance will cause the capacitorto charge up linearly at a rate proportional to the intensity ofillumination on photomultiplier 30. Since, however, the voltage on thexerographic plate decays in an approximately exponential fashion underillumination, it is preferable to employ a lower value of amplifier gainand a lower value of resistance, such that capacitor 38 is chargedexponentially in a manner approximating the decay of voltage on thexerographic plate. After exposure is terminated, the developmentelectrode is swung down into position and switch 28 is moved to theright-hand position so that the formerly grounded side of the capacitor38 is now ungrounded and connected to the development electrode 24 andthe side of the capacitor form- In the illustrated embodiment, thepolarity of power supply 40 will be the same as the polarity of chargeon the xerographic plate 12 and similarly, the output of amplifier 34will be of that same polarity. Accordingly, when switch 28 is moved tothe right-hand position, the potential on the development electrode 24will be equal to the potential of power supply 40 minus the potentialaccross capacitor 38, and will closely approximate the potential whichappears on plate 12 in areas corresponding to image highlights or whichwould appear as such highlights. When development is thereupon carriedout by the previously described methods, or any other methods, therewill be no electric field between the xerographic plate and thedevelopment electrode in image highlight areas and no electrostaticallyattractable particles will be deposited on the xerographic plate, oreven on the development electrode, in such areas. The result will be adeveloped image having clear, background free highlights or backgroundareas, and this result will be achieved automatically over a wide rangeof exposures.

Where it is known that the original subject 19 will have white marginsor other areas which will be predictably of lowest density, samplematerial 32 may be omitted and the photomultiplier adjusted so as tofocus on such margin areas. In such cases, it will generally not befeasible to aim photomultiplier 30 through lens 22 since thephotomultiplier would then have to occupy the same position as thexerographic plate. However, the photomultiplier can then be providedwith a lens system of its own. FIG. 2 shows a simplified form of part ofthe apparatus of FIG. 1 in which the photomultiplier 30, amplifier 34,and resistor 36 have been replaced by a power supply 42 and a pair ofvariable resistors 44 and 46. One of the resistors is calibrated interms of lens aperture and the other resistor is calibrated in terms ofbackground optical, density of the subject being reproduced. A relay 48connects the power supply only when the lamps are lit. In this way, thepotential developed across capacitor 38 will be a function of the lensaperture, the background density, and the length of exposure and willaccurately reflect the decrease in potential on the xerographic plate.However, the apparatus of this figure, unlike that of FIG. 1, will beaffected by such factors as variations in the output of lamps 20 due toaging or variations in supply voltage as well as to the affects of dustand dirt on lens 22. The illustrated arrangement, however, is adequatefor many purposes and may be substituted for the photopulllltipliershown in FIG. 1 or in any of the figures which FIG. 3 shows a furtherembodiment of the invention in which the initial potential on thexerographic plate is measured before each exposure. This will oftenprove desir-able because existing electrostatic charging apparatussometimes produces variable results from one charging to another andbecause different xerographic plates will often accept varyingpotentials from the same charging apparatus. In this embodiment, switch50 is first placed in the left-hand position, which causes solenoid 52to raise an electrometer 54 into a position where it is adjacent to thesurface to the photoconductive insulating surface 16 of the xerographicplate and produces an output voltage proportional to such potential. Atthe same time, relay 56 is closed, connecting the electrometer throughinput resistor 58 to operational amplifier 60 which amplifies theelectrometer output by a factor equal to the ratio of feedback resistor62 to input resistor 58. When switch 50 1s placed in the intermediateposition, at or prior to exposure, the electrometer is retracted fromthe exposure path and relay 56 is opened, thus causing the operationalamplifier 60, in conjunction with feedback capacitor 64, to hold itsprevious output. The operational amplifier and its associated componentswill be recognized as constituting a conventional sample and holdcircuit. During exposure, the circuit of this figure functions exactlythe same as that of FIG. 1 and after exposure, switch 50 is turned tothe right-hand position and the circuit again functions as in FIG. 1,except that the power supply 40 of FIG. 1 has been replaced by theactual measured initial plate potential as stored at the output ofamplifier 60.

In this figure, a different development arrangement is exemplifiedwherein capacitor 38 is connected to a development electrode remote fromsupport member 10. After exposure, the xerographic plate is removed fromsupport member and placed on a second support member 66 which ispositioned adjacent to development electrode 24. A developer hopper 68is positioned above the channels formed between the developmentelectrode and the xerographic plate and is provided with a solenoidvalve 70 such that when switch 50 is turned to the righthand ordevelopment position, the solenoid valve will open and dischargedeveloper material across the xerographic plate.

FIG. 4 shows another form of the invention, the mechanical structure ofthe previous figures being omitted for simplicity. In this embodiment,one end of capacitor 38 is permanently grounded or connected to a sourceof fixed potential. The other end of the capacitor is initiallyconnected through switch 72 to a source of fixed potential 74 and isthereafter discharged by a photomultiplier 30 acting through amplifier34 and resistor 36 by an amount equal to the loss in potential sufferedby the xerographic plate itself. A buffer-amplifier 76 may be used inthis figure or in the embodiments of other figures to transfer thesupply voltage to the development electrode, where the capacitance orleakage resistance of the electrode would otherwise discharge thecapacitor 38. If the bufferamplifier has a gain other than unity, thevoltage developed on capacitor 38 need not be equal to the desireddevelopment electrode potential but need only be proportional desiredvalue. Since the voltage on the capacitor can be made opposite to thepolarity of the residual plate potential, the same results as previouslydescribed may be had by applying the resulting voltage to the back ofthe xerographic plate, and grounding the development electrode,providing exactly the same fields between the plate and developmentelectrodes as in the previous emboditments. I

FIG. 5 shows an embodiment similar to that of FIG. 4 except that anelectrometer 54 is employed to measure the actual initial potential onthe xerographic plate. Since this potential is only required to be usedto precharge capacitor 38 before exposure, only a simple amplifier 60 isrequired rather than the sample and hold circuit of FIG. 3.

FIG. 6 shows a final and preferred embodiment of the invention. Prior toexposure, capacitor 38 is charged to the initial plate potential byelectrometer 54 and amplifier 60. During exposure, capacitor 38 isconnected to a variable resistance circuit 78 which is controlled byphotomultiplier 30. More specifically, resistance circuit 78 provides acontrollable resistance in shunt with capacitor 38 and which isinversely proportional to the light incident on photomultiplier 30during exposure. Thus, capacitor 38 will be discharged in an exponentialfashion with a time constant which is a function of the incidentillumination. This corresponds quite closely to the way the xerographicplate itself is affected by radiation and, accordingly, the potential oncapacitor 38 at the end of exposure will accurately reflect thepotential on the xerographic plate in background or highlight areas.This potential can be applied to the development electrode exactly as inprevious figures.

FIG. 7 illustrates one way in which the variable resistance circuit 78of FIG. 6 can be realized. The capacitor is connected through acontrollable resistive device 80, such as a transistor or vacuum tube,to a current sampling resistor 82 and to a high impedancebuffer-amplifier 84, such as a field-effect transistor in asourcefollower configuration. A sample of the buffer amplifier output istaken by a voltage divider comprising resistors 86 and 88 and comparedin a dividing circuit 90 with the voltage across current samplingresistor 82. The output of dividing circuit 90 will represent the ratioof the capacitor voltage to the current drawn from it and hence will beproportional to the apparent resistance in parallel with the capacitor.Preferably, the inputs to the divider circuit are connected so that theoutput is instead proportional to one over the effective resistance. Theoutput of the divider circuit is compared with the photomultiplieroutput in a differential amplifier 92 which generates an output signalproportional to the difference between the resistance seen by capacitor38 and the desired resistance. This signal is applied to the controlterminal (e.g., base or grid) of the resistive element 80. There is thusformed a feedback regulating circuit which constrains the resistiveelement to simulate a variable resistor having a value inverselyproportional to the light intensity at the photomultiplier.

The foregoing figures and the description thereof are intended to beillustrative of the invention ratherthan definitive thereof.Accordingly, the scope of the invention is to be determined solely fromthe claims, as many variations of the foregoing figures and descriptionswill be obvious to anyone skilled in the art.

What is claimed is:

1. In a xerographic exposure and development apparatus wherein axerographic plate electrostatically charged to an initial potential isexposed to an image pattern of radiation and thereafter developed bycontact with electroscopic particles while in close proximity to adevelopment electrode and as a function of the electric field betweensaid plate and electrode;

the developed image background minimizing improvement comprising:

first circuit means activated solely during exposure to generate apotential increment which is a direct function of exposure time andexposure intensity and approximates the plate voltage decrement inmaximally exposed areas thereof, and second circuit means to substractsaid increment from said initial potential and apply the resultingpotential between said plate and electrode to eliminate the fieldtherebetween adjacent maximally exposed areas of the plate.

2. The apparatus of claim 1 wherein said initial potential is measuredbefore each exposure.

3. The apparatus of claim 1 wherein said initial potential isrepresented by a fixed potential.

4. The apparatus of claim 1 in which said first circuit comprises aphotodetector exposed to said image pattern of radiation and connectedto one terminal of a storage capacitor, the other terminal of saidstorage capacitor being connected to one of said plate and electrode,and said second circuit comprises a switch to disconnect said capacitorfrom said photodetector and connect it to said initial potential.

5. The apparatus of claim 1 in which said first circuit comprises afixed potential, a variable resistance means connected to said fixedpotential, said resistance means being calibrated so that the potentialdeveloped thereacross is a function of the exposure intensity andduration and background density, said resistance means being connectedto one terminal of a storage capacitor, the other terminal of saidstorage capacitor being connected to one of said plate and electrode,and said second circuit means comprising a switch to disconnect saidcapacitor from said resistance means and connect it to said initialpotential.

6. The apparatus is defined in claim 2 wherein said first circuitcomprises a photodetector exposed to said image pattern of radiation andconected alternately to one terminal of a storage capacitor and to theoutput of a sample and hold circuit, the other terminal of said storagecapacitor being connected to provide a charging path for said capacitor,and said second circuit comprises a switch to disconnect said capacitorfrom said photodetector and connect it to the sample and hold circuitand to disconnect the other terminal of said capacitor from saidcharging path and connect it to one of said plate and electrode.

7. The apparatus of claim 2 wherein said first circuit comprises aphotodetector exposed to said image pattern of radiation and connectedto one input of a variable resistance circuit, the output of saidvariable resistance circuit being connected to one terminal of a storagecapacitor, said terminal also being connected to one of said plate andelectrode, and said second circuit comprising a switch to disconnectsaid capacitor from said variable resistance and connect said initialpotential thereto.

8. The apparatus as defined in claim 7 where it said variable resistancecircuit comprises a controllable resistive device, means connected tosaid device and to said storage capacitor for producing an output whichis pro portional to the apparent resistance in parallel with saidcapacitor, means connected to the output of said proportional outputproducing means for comparing the output thereof and the output of saidphotodetector, and

means for connecting the output of said comparator means to the input ofsaid controllable resistive device. 9. The apparatus as defined in claim8 wherein said controllable resistive device comprises a transistor.

References Cited UNITED STATES PATENTS 3,241,466 3/1966 Clark.

NORTON ANSHER, Primary Examiner.

LEO H. MCCORMICK, Assistant Examiner

